One important step in the manufacture of printed circuit boards is the plating of conductive material (typically, e.g., copper) onto a substrate to provide the desired circuit paths for the board. Usually the circuit board substrate will include holes extending through the thickness of the board. When plated, the holes connect the traces on one side of the board to traces on the other side of the board.
As electronic components get smaller, and the circuit traces on circuit boards get thinner (so as to provide more circuit density on a board), it becomes desirable to make the diameter of holes also proportionately smaller. As the size of the holes decrease, however, difficulties arise in obtaining the desired thickness of the copper plating within the hole. That is, the thickness of the copper deposited inside the holes tends to decrease (in comparison to the thickness deposited on adjacent circuit traces on the surface of the circuit board) as the hole diameter decreases.
This problem is fairly well-known, and one solution that is commonly employed to improve plating results is employing reverse pulsed DC power. In this technique, a DC current of, e.g., a few volts and about 300 amps is applied in the forward direction for, e.g., a few milliseconds, and then is reversed, typically at a higher amperage (e.g., about 900 amps) for a shorter time (e.g., a few microseconds). This pulsing has the effect of increasing the amount of material plated within the holes. With such rapid pulsing, however, inductance in the system prevents the actual waveform from being as square as would be desirable. That is, once current reversal is initiated, it takes some time to build the reverse current--this time is effectively wasted, and becomes a limiting factor on the frequency at which polarity reversal can be accomplished (higher frequencies being more effective and, therefore, desired). Thus, it is desirable to reduce the inductance in the overall system.
There are practical physical limitations on the configuration of plating systems, however. For example, in many plating line installations, the power supplies are mounted at one end of the plating tanks. Manufacturers prefer to have both ends of the anodes and cathodes supplied with current in an attempt to ensure even electrical distribution. This creates a problem in that one end of the anode may be, e.g., five feet from the power supply while the other end may be twenty feet away. The runs of cable to each end must be of the same length, however; if one were shorter than the other, it would have less resistance (a significant issue when working voltage is only a few volts), producing an uneven distribution of voltage on the anode and cathode and potentially overheating the shorter cable. The typical solution to this problem is to make both cables of the same length and simply coil the excess length of the cable that connects to the near end of the tank. Such lengths of cable, however, increase the inductance of the system, and therefore reduce the effectiveness of the reverse pulse technique.